1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to a method and apparatus for providing low velocity retraction of an actuator through application of an exponentially ramped voltage to a disc drive voice coil motor.
2. Discussion
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted for movement to an electronically controlled actuator mechanism.
Presently, the most commonly used type of actuator mechanism is the rotary voice coil actuator, which employs a voice coil motor. With this type of actuator, the heads used to write and read data are mounted via flexures at the ends a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body is journaled via ball bearing assemblies to rotate about a pivot shaft which is mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs and the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled DC current is passed through the coil, a magnetic field is set up which interacts with the magnetic field of the permanent magnets and causes the coil to move relative to the permanent magnets in accordance with the well-known Lorentz relationship. As the coil moves relative to the permanent magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Control of the movement of the heads is achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy, McKenzie, Heydari and Woods, assigned to the assignee of the present invention and incorporated herein by reference. In such a system, position or servo information is prerecorded on at least one surface of one of the discs. The servo system can be either a dedicated servo system, in which servo information is provided on a dedicated servo disc surface, or an embedded servo system, in which servo information is interleaved with user data on all the disc surfaces.
At such times that the disc drive is not in use, the heads are usually "parked", or moved to a position over a designated park zone which is generally at an inner radius of the discs. Heads are generally parked in response to a loss of power to the drive, or in response to a "sleep" command which is issued to conserve power, an important consideration for drives used in portable computers. In order to park the heads, a current is provided to the coil in order to move the actuator across the disc and over the park zone, where a latch is provided to secure the actuator. Typical latching arrangements are disclosed by U.S. Pat. No. 5,187,627 entitled MAGNETIC LATCH AND CRASH STOP, issued to Hickox and Stram, U.S. Pat. No. 5,224,000 entitled CRASH STOP AND MAGNETIC LATCH FOR A VOICE COIL ACTUATOR, issued to Casey and West, and U.S. Pat. No. 5,365,389 entitled CRASH STOP AND MAGNETIC LATCH, issued Nov. 15, 1994 to Jabbari and Tafreshi, all of these references being assigned to the assignee of the present invention and incorporated herein by reference.
It is generally desirable to park the heads quickly, as the power to park the heads at the time of a power loss is typically provided from the back EMF of the spindle motor; that is, after a loss of power, the spindle motor is used as a generator as it continues to spin due to inertia. As areal densities increase and the size of the discs continue to decrease, the amount of power available from the inertia of the spinning spindle motor after power down will continue to decrease, making expeditious parking of the heads an even more important consideration in the future. The general use of the spindle motor back EMF to provide power used during a disc drive shut down sequence is discussed in U.S. Pat. No. 4,679,102 entitled METHOD AND MEANS FOR PARKING READ/WRITE HEADS IN A DISC DRIVE USING THE BACK-EMF OF THE SPINDLE MOTOR TO OPERATE A STEPPER MOTOR, issued to Wevers, Krause and Battu, assigned to the assignee of the present invention and incorporated herein by reference.
Additionally, the heads are typically supported over the discs by an air-bearing surface generated by the rotation of the discs and this air-bearing surface will be lost once the rotational speed of the discs is sufficiently decreased. Typically, the heads are parked by applying a DC current pulse to the coil in order to move the actuator to the park position. The duration and magnitude of the current pulse are selected in order to ensure the heads are reliably parked from any position on the disc, including the outermost radius of the disc.
While it is desirable to park the heads quickly, attendant problems have been observed when the actuator is moved too rapidly during parking. Particularly, the application of a current pulse to the voice coil motor to park the heads can result in significant acceleration of the heads towards the park position, culminating in undesirable acoustic noise as the actuator contacts a limit stop or magnetic latch. Further, the sudden deceleration of the actuator upon contact with the limit stop can subject the heads to uncontrolled mechanical motion and vibration, which can damage the heads, the actuator arms and the disc media. With the advent of smaller, more complex head designs, such as MR and GMR heads, reducing mechanical motion and shock to the heads will continue to be desirable in the future.
Uncontrolled bias forces, if not compensated, can compound the problems associated with the parking of the heads. Aerodynamic forces on the heads as well as bias forces from a flex circuit connected to the actuator (to provide both data signal and drive signal paths) vary across the disc radius and may cause additional acceleration of the heads toward the park position, causing additional acoustic noise and head motion when the actuator is parked. Further, electrical and mechanical tolerances in the components used in the actuator and associated control circuitry can also result in variations in parking characteristics from drive to drive, which can require individual turning of drives during manufacturing to provide acceptable parking characteristics.